This invention relates to a selective etching method of a polyimide type resin film. More particularly, the present invention relates to an etching method of a polyimide type resin film used as an interlevel insulation film of multilevel metallization or a passivation film of transistors, ICs and LSIs.
The conventional etching method of the polyimide type resin film can be broadly classified into wet etching and dry etching. Wet etching can be further classified into a method to be practised under the state in which the polyimide type resin is baked at a temperature ranging from about 100.degree. to about 150.degree. C. and is semicured (B-stage) and another which is to be practised after the polyimide resin is cured. The etching method to be carried out under the semicured state of the polyimide resin makes use of a positive type photoresist consisting of a Novolac type resin soluble to an alkaline solution such as AZ1350 (tradename of Shipley Co., U.S.A.) or OFPR (tradename of Tokyo Oka K.K., Japan) and etches the polyimide type resin film simultaneously with developing the former resin using the developing solution. However, the developing solution is alkaline and is permeable to the positive type photoresist film so that etching can not be made with a high level of accuracy in forming a fine via hole as small as 10 .mu.m square.
On the other hand, it is also possible to practise etching under the same semicured state by use of a negative photoresist such as OMR 83 (tradename of Tokyo Oka K.K.) of a cyclic rubber type in a mixed solution of hydrazine hydrate and a polyamine such as ethylenediamine. However, the etching speed of the polyimide film is extremely susceptible to the baking temperature during semicuring and a problem is yet left as to the stability of etching.
Among the conventional etching methods, the most stable etching method is to carry out etching by use of a mixed solution of hydrazine hydrate and a polyamide such as ethylenediamine under the state in which the polyimide film is fully cured at a temperature of about 350.degree. C. A cyclic rubber-bisazide type negative photoresist such as OMR 83 is selected as the photoresist because it is only one resist durable to hydrazine hydrate, and a fine via hole as small as 3 .mu.m square can be made.
The dry etching method uses O.sub.2 plasma or O.sub.2 sputtering. However, since this method requires an additional masking the film such as Mo or Cr or other inorganic films, it involves the drawback that the number of etching steps increases.
The following references are cited to show the state of the art:
______________________________________ (1) U.S. Pat. No. 3,846,166 to A. Saiki et al; (2) U.S. Pat. No. 4,113,550 to A. Saiki et al; (3) U.S. Pat. No. 4,218,283 to A. Saiki et al. ______________________________________